The invention relates generally to optical amplifiers and, more particularly, to lightwave systems and networks utilizing such amplifiers.
Optical amplifiers are commonly used in lightwave communication systems as in-line amplifiers for boosting signal levels to compensate for losses in a transmission path, as power amplifiers for increasing transmitter power, and as pre-amplifiers for boosting signal levels before receivers. In wavelength division multiplexed (WDM) systems, which combine many optical channels at different wavelengths for transmission as a composite signal in an optical fiber, optical amplifiers are particularly useful because of their ability to amplify all channels simultaneously.
Erbium-doped fiber amplifiers are predominantly used in current WDM communication systems because of their gain characteristics and ease of coupling with optical fiber. Erbium-doped fiber amplifiers are particularly desirable for intensity modulated digital optical communication systems, wherein the light intensity of signal channels is modulated to represent the xe2x80x9c1xe2x80x9ds and xe2x80x9c0xe2x80x9ds of digital data. In particular, slow gain dynamics allow erbium-doped fiber amplifiers to provide constant gain to all signal channels in a WDM system regardless of bit transitions in the intensity modulated bit patterns. However, despite their usefulness in long haul transmission applications, the disadvantages of erbium-doped fiber amplifiers are well known. For example, erbium-doped fiber amplifiers are expensive and, as a result, do not provide the most cost effective solution for applications such as metropolitan optical networking and the like. Moreover, erbium-doped fiber amplifiers have a relatively narrow usable gain bandwidth which will become more of a problem in emerging long haul systems which have higher channel counts and which will use new optical fiber having a wider usable bandwidth.
By contrast, semiconductor optical amplifiers are comparatively inexpensive, have a large gain bandwidth, and can be easily integrated with other devices. However, semiconductor optical amplifiers have several limitations which have limited their use in optical communication systems to date. In particular, the fast gain dynamics and nonlinear gain characteristics of semiconductor optical amplifiers can be problematic. For example, gain changes quickly as input power changes and is not constant for the modulation speed of current communication systems, thus resulting in problems such as inter-modal distortion and saturation-induced crosstalk, i.e., cross-saturation.
Briefly, cross-saturation results when intensity modulation in one channel leads to modulation of the gain available for other channels. For example, the gain of a specific channel is saturated not only by its own power, but also by the power of the other channels in the system. Cross-saturation is particularly problematic in intensity modulated systems because the channel power changes with time depending on the bit pattern. The signal gain of one channel then changes from bit to bit, and the change depends on the bit patterns of the other channels. Such gain fluctuations can result in detection errors which degrade overall bit error rate performance.
Gain control schemes, such as feedforward or feedback gain control loops, gain clamping, and pump light injection schemes, have been proposed for reducing the effects of inter-modal distortion and cross-saturation. See, e.g., U.S. Pat. No. 5,017,885, entitled xe2x80x9cOptical Amplifier with Reduced Nonlinearityxe2x80x9d, issued May 21, 1991 to A. Saleh, U.S. Pat. No. 5,576,881, entitled xe2x80x9cMulti-Frequency Optical Signal Source Having Reduced Distortion and Crosstalkxe2x80x9d, issued Nov. 19, 1996 to Doerr et al., Simon et al., xe2x80x9cTravelling Wave Semiconductor Optical Amplifier with Reduced Nonlinear Distortionsxe2x80x9d, Electronics Letters, vol. 30, no. 1, January 1994, Tiemeijer et al., xe2x80x9cReduced Intermodulation Distortion in 1300nm Gain-Clamped MQW Laser Amplifiersxe2x80x9d, IEEE Photonics Technology Letters, vol. 7, no. 3, March 1995, and Yoshino et al., xe2x80x9cImprovement of Saturation Output Power in a Semiconductor Laser Amplifier Through Pumping Light Injectionxe2x80x9d, IEEE Photonics Technology Letters, vol. 8, January 1996, each of which is incorporated by reference herein. Among other disadvantages, these gain control schemes add cost and complexity to the system, e.g., because of additional circuitry for feedback or feedforward loops and the like.
Alternatively, inter-modal distortion and cross-saturation may be reduced by operating optical amplifiers in the small-signal region, i.e., unsaturated region. However, for practical applications, it is desirable to operate optical amplifiers in the saturation region to achieve high output power and other efficiencies. For example, WDM systems typically operate in the saturation region because of the high output power needed for wide dynamic range and high signal to noise ratios. Accordingly, inter-modal distortion and cross-saturation are still a problem for systems having optical amplifiers operating in the saturation region.
Distortion and crosstalk that occurs when operating optical amplifiers in saturation is substantially reduced according to the principles of the invention by passively compensating for gain variations caused by changes in input power to the optical amplifiers. More specifically, in an optical communication system having one or more optical amplifiers, passive gain control is achieved by supplying at least one optical channel in addition to the other traffic-carrying optical channels, wherein the additional optical channel absorbs or otherwise receives most of the gain variations while the traffic-carrying channels experience less.
Because optical channels having wavelengths near the gain peak region in an optical amplifier""s gain bandwidth typically suffer the highest gain variations and are most susceptible to gain-induced crosstalk, the additional optical channel in one exemplary embodiment is assigned a wavelength at or near the gain peak region where the gain variations are at a maximum. By appropriate selection of wavelength and initial power of the additional optical channel, the power level in the additional optical channel rises and falls in response to changes in power levels of the traffic-carrying channels. As such, the additional optical channel serves as a xe2x80x9creservoirxe2x80x9d channel that compensates for gain variations caused by changes in input power to the optical amplifier.
According to one exemplary embodiment, a wavelength division multiplexed (WDM) signal having a plurality of optical channels of respective wavelengths is amplified by one or more semiconductor optical amplifiers in a WDM system. A reservoir channel is inserted prior to the first semiconductor optical amplifier at a wavelength that is located at or near the point of maximum gain variation, e.g., typically the shorter wavelength region in the gain spectrum of the semiconductor optical amplifiers. As input power to the semiconductor optical amplifiers varies, e.g., as power levels in the incoming traffic-carrying WDM optical channels fluctuate, the gain variations in the semiconductor optical amplifiers will be highest where the reservoir channel is located. As such, the reservoir channel will experience the highest amount of gain variation and, as a result, will passively compensate for the distortion and crosstalk that would otherwise occur in the traffic-carrying optical channels.
Because effects of inter-modal distortion and cross-saturation are substantially reduced according to the principles of the invention, a system having optical amplifiers operating in saturation can therefore achieve substantial improvements in bit error rate performance as compared with prior art schemes. Moreover, because passive compensation is used instead of active feedback and feedforward control schemes as in the prior art, cost and complexity is substantially reduced. Consequently, such a solution can be readily implemented and advantageously used, especially in metropolitan area optical networking applications where cost is a primary consideration.